Effect of carbon dioxide enrichment on physico-chemical and enzymatic changes in tomato fruits at various stages of maturity

The influence of CO₂ enrichment on fruit growth, firmness and colour, together with its effect on the concentrations of ascorbic acid, organic acids and sugars, and the activities of sucrose synthase (SS) (UDP glucose: D-fructose 2-glucosyltransferase, E. C. 2. 4. 1. 13) and sucrose phosphate synthase (SPS) (UDP glucose: D-fructose-6-phosphate 2-glucosyltransferase, E. C. 2. 4. 1. 14) were determined at various stages of maturity in fruits of tomato (Lycopersicon esculentum Mill. cv. Momotaro). CO₂ enriched tomatoes had lower amounts of citric, malic and oxalic acids, and higher amounts of ascorbic acid, fructose, glucose and sucrose synthase activity than the control. Elevated CO₂ enhanced fruit growth and colouring during development. Citric acid was the primary organic acid followed by malic and oxalic acids. The concentration of organic acids (mg g^-1 fresh weight) and of ascorbic acid (mg 100g^-1 fresh weight) increased with the maturity of fruits; their maximum concentrations were found at the pink stage of ripening, but declined slightly at the red stage. The amount of reducing sugars (mg g^-1 fresh weight) increased with the advancement of maturity, with fructose being the predominant sugar. The decrease in SS activity was accompanied by an increase in the concentrations of reducing sugars. There were no significant differences in fruit firmness, sucrose concentration and SPS activity between the treatments. The SPS activity did not change, but remained relatively constant throughout fruit development. The results also suggest that SS levels correlated positively with sucrose concentration but negatively with the concentration of reducing sugars.